<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>sameheight</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        .content{
            width: 900px;
            border: 1px solid #acf;
            overflow: hidden;
        }
        .clearfix:after{
            content: "";
            display: block;
            clear: both;
        }
        .left,.right{
            float: left;
            width: 400px;
            background-color: #ccc;
            padding-bottom: 1000px;
            margin-bottom: -1000px;
            border: 1px dashed #aaa;

        }
    </style>
</head>
<body>
    <div class="content clearfix">
        <div class="left">
            a <br>
            a <br>
            a <br>
            a <br>
            a <br>
            a <br>
            a <br>
            a <br>
            a <br>
        </div>
        <div class="right">
                a <br>
                a <br>
                a <br>
                a <br>
                a <br>
        </div>
    </div>
</body>
</html>